Error: cannot fetch last explain plan from PLAN_TABLE

最近遇到了错误“Error: cannot fetch last explain plan from PLAN_TABLE”，于是稍微研究了一下哪些场景下碰到这种错误，具体参考下面案例：

1：忘记使用EXPLAIN PLAN放在SQL语句前面，然后使用使用SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY)查看具体SQL的执行计划时，就会遇到错误“Error: cannot fetch last explain plan from PLAN_TABLE”。如下所示：

SQL> show user;

USER is "SYS"

SQL> SELECT *

  2  FROM SCOTT.EMP

  3  WHERE HIREDATE BETWEEN '01-JAN-1981' AND '01-APR-1981';

     EMPNO ENAME      JOB              MGR HIREDATE         SAL       COMM     DEPTNO

---------- ---------- --------- ---------- --------- ---------- ---------- ----------

      7499 ALLEN      SALESMAN        7698 20-FEB-81       1600        300         30

      7521 WARD       SALESMAN        7698 22-FEB-81       1250        500         30

SQL> COL PLAN_TABLE_OUTPUT FOR A180;

SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------

Error: cannot fetch last explain plan from PLAN_TABLE

其实，这种情形是因为SQL语句中忘记使用EXPLAIN PLAN，一般而言EXPLAIN PLAN会将SQL对应的执行计划放入plan_table。官方文档介绍如下：

The EXPLAIN PLAN statement displays execution plans chosen by the Oracle optimizer for SELECT, UPDATE, INSERT, and DELETE statements. A statement's execution plan is the sequence of operations Oracle performs to run the statement. The row source tree is the core of the execution plan.

如果没有使用EXPLAIN PLAN，那么没有将对应SQL的执行计划放进PLAN_TABLE，而如果使用EXPLAIN PLAN，那么ORACLE会用格式化的数据填充PLAN_TABLE表，以便以易读的格式呈现给用户。个人使用10046跟踪对比了一下（对比使用EXPLAIN PLAN和不使用EXPLAIN PLAN两种情况），使用EXPLAIN PLAN时，数据库会向plan_table插入数据。如下所示：

2：对应的用户下存在PLAN_TABLE表（这个可能情况比较复杂），然后使用ALTER SESSION SET CURRENT_SCHEMA设置当前会话的SCHEMA时可能会遇到这种场景。

在SCOTT用户下创建一个PLAN_TABLE（结构一样，如果结构不一样，会报另外一种错误）

SQL> SHOW USER; 

USER is "SCOTT"

SQL>CREATE TABLE PLAN_TABLE AS 

  SELECT STATEMENT_ID, 

         PLAN_ID, 

         TIMESTAMP, 

         REMARKS, 

         OPERATION, 

         OPTIONS, 

         OBJECT_NODE, 

         OBJECT_OWNER, 

         OBJECT_NAME, 

         OBJECT_ALIAS, 

         OBJECT_INSTANCE, 

         OBJECT_TYPE, 

         OPTIMIZER, 

         SEARCH_COLUMNS, 

         ID, 

         PARENT_ID, 

         DEPTH, 

         POSITION, 

         COST, 

         CARDINALITY, 

         BYTES, 

         OTHER_TAG, 

         PARTITION_START, 

         PARTITION_STOP, 

         PARTITION_ID, 

         TO_LOB(OTHER) AS OTHER, 

         OTHER_XML     AS OTHER_XML, 

         DISTRIBUTION, 

         CPU_COST, 

         IO_COST, 

         TEMP_SPACE, 

         ACCESS_PREDICATES, 

         FILTER_PREDICATES, 

         PROJECTION, 

         TIME, 

         QBLOCK_NAME 

  FROM   PLAN_TABLE;

SQL> EXPLAIN PLAN FOR

  2  SELECT * FROM DUAL;

Explained.

SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY); #SCOTT用户下不会出错。

PLAN_TABLE_OUTPUT

----------------------------------------------------------------------------

Plan hash value: 272002086

--------------------------------------------------------------------------

| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |

--------------------------------------------------------------------------

|   0 | SELECT STATEMENT  |      |     1 |     2 |     2   (0)| 00:00:01 |

|   1 |  TABLE ACCESS FULL| DUAL |     1 |     2 |     2   (0)| 00:00:01 |

--------------------------------------------------------------------------

8 rows selected.

SQL> 

但是我们使用ALTER SESSION SET CURRENT_SCHEMA设置当前会话的SCHEMA后，那么再按之前的SQL测试，就会遇到这个错误，如下所示：

SQL> show user;

USER is "SYS"

SQL> alter session set current_schema=SCOTT;

Session altered.

SQL> EXPLAIN PLAN FOR

  2  SELECT *

  3  FROM SCOTT.EMP

WHERE HIREDATE BETWEEN '01-JAN-1981' AND '01-APR-1981';  4  

Explained.

SQL> COL PLAN_TABLE_OUTPUT FOR A180;

SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

Error: cannot fetch last explain plan from PLAN_TABLE

SQL> SET LINESIZE 1200

SQL> SELECT OWNER,OBJECT_NAME,OBJECT_TYPE,CREATED FROM ALL_OBJECTS 

  2  WHERE OBJECT_NAME LIKE 'PLAN_TABLE%' 

  3  AND OWNER IN (SYS_CONTEXT('USERENV','CURRENT_SCHEMA'),'PUBLIC','SYS');

OWNER                          OBJECT_NAME                    OBJECT_TYPE         CREATED

------------------------------ ------------------------------ ------------------- ---------

SYS                            PLAN_TABLE$                    TABLE               24-MAY-15

PUBLIC                         PLAN_TABLE                     SYNONYM             30-JUN-05

SCOTT                          PLAN_TABLE                     TABLE               21-DEC-19

如果遇到这种情况，可以使用上面脚本看看是否存在同名的PLAN_TABLE，这种情况下，可以将SCOTT下的PLAN_TABLE表重命名或删除即可。当然也可以用下面方法

SQL> EXPLAIN PLAN INTO SCOTT.PLAN_TABLE FOR

  2  SELECT *

  3  FROM SCOTT.EMP

WHERE HIREDATE BETWEEN '01-JAN-1981' AND '01-APR-1981';

Explained.

SQL> COL PLAN_TABLE_OUTPUT FOR A180;

SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);

PLAN_TABLE_OUTPUT

---------------------------------------------------------------------------------------

Plan hash value: 3956160932

--------------------------------------------------------------------------

| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |

--------------------------------------------------------------------------

|   0 | SELECT STATEMENT  |      |     2 |    74 |     2   (0)| 00:00:01 |

|*  1 |  TABLE ACCESS FULL| EMP  |     2 |    74 |     2   (0)| 00:00:01 |

--------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

PLAN_TABLE_OUTPUT

------------------------------------------------------------------------------------------------------------

   1 - filter("HIREDATE"<=TO_DATE(' 1981-04-01 00:00:00', 'syyyy-mm-dd

              hh24:mi:ss') AND "HIREDATE">=TO_DATE(' 1981-01-01 00:00:00',

              'syyyy-mm-dd hh24:mi:ss'))

15 rows selected.

SQL> 

当然，还可以更深入的探究，只是没有太多价值，而且个人在测试过程中，发现还有许多其它状况，例如解决了这个错误后，再去测试，就发现不报错。但是显示的执行计划还是原来SQL（不是当前SQL的执行计划）......... 。当然也不排除还有一些场景可能遇到这个错误。这里仅仅描述了两种场景。